1. Field of the Invention
This invention relates generally to controlling an automatic transmission downshift during a coasting drive condition when a tie-up or neutral case occurs as evidenced by measured output torque.
2. Description of the Prior Art
Friction elements, i.e., the clutches and brakes, are utilized to alter a torque path within the gear sets to establish desired torque and speed ratios between input and output of an automatic transmission system. During a vehicle operation, a combination of engaged friction control elements determines a specific gear position in which an automatic transmission operates. When the vehicle is in a coasting condition wherein the vehicle continues to move forward while both acceleration pedal and brake pedal are released, a transmission controller may initiate the so called coasting downshift as the vehicle slows down. During the downshift process, some friction elements, referred to as off-going elements, are released while others, referred to as on-coming elements, are engaged, altering torque and speed relationship between input and output of the transmission.
A neutral state, i.e., a state in which no torque path is dominantly established within the gear sets, is caused by premature release of an offgoing control element or lack of oncoming control element torque capacity. When the neutral state occurs during a coasting downshift, a driver may perceive a shift shock or a momentary loss of drive torque. Additionally, if the driver steps in an acceleration pedal during the neutral state, engine speed may surge, followed by a shift shock due to a sudden engagement of on-coming element. The neutral state is not generally detectable by means of the sensors, such as transmission speed sensors, that are commonly available in volume production vehicles.
A transmission tie-up at a level perceptible to the driver may be caused by delayed release of the offgoing control element or torsional overcapacity of the oncoming control element, causing the gear sets to be over-constrained. When the tie-up state occurs during the coasting downshift, a driver may perceive a shift shock or a sudden loss of drive torque. A severe tie-up state may be detected by observing a measurable drop of transmission input speed signals in volume production vehicles. When the severe tie-up is detected, a controller may quickly reduce off-going clutch pressure to resolve the condition.
According to prior art coasting shift control methodologies, control pressure of off-going element is reduced through an open-loop approach while control pressure of on-coming clutch is raised through an open-loop approach. Open-loop pressure controls for both off-going clutch and on-coming clutch may be adaptively adjusted. That is, after completing each shift event, a transmission controller may adjust open-loop pressure profile for off-going element by a pre-determined amount to delay its release timing while prescribing faster pressure rise for on-coming clutch element by a pre-determined amount. This adjustment brings the coasting shift control toward tie-up state. When the tie-up is detected through speed measurements, the controller may reduce off-going element control pressure for its immediate release within the given shift. The detection of the tie-up state enables the controller to adaptively prescribe the control pressure profiles for avoiding both neutral and tie-up states in the subsequent coasting down-shift events.
The prior art methodologies that primarily rely on speed measurements can neither explicitly detect nor correct the neutral state within the current coasting downshift event. The prior art methodologies that primarily rely on speed sensors may detect the tie-up state, but does not provide means to control the amount of dropped drive torque level during the coasting downshift event. The adaptive open-loop pressure adjustments may not work well because off-going and on-coming clutch friction torques may unpredictably vary under different operating conditions even if control pressure profiles remain unchanged. The prior art adaptive pressure adjustments may not be able to account for changing hydraulic control system variability. Accordingly, even if optimal pressure profiles are adaptively identified, they may not be optimal for the subsequent shifts and neutral or tie-up state may still occur.